1. Field of the Invention
The invention relates to the blending of particulate solids and in particular to a method and apparatus for converting a heterogeneous mixture of solid UO.sub.2 powders to a homogeneous mixture.
2. Description of the Prior Art
The blending of particulate solids has been accomplished in the past in a variety of ways. Mechanical mixers of several types, such as tumble mixers, ribbon blenders and high shear mixers have been used. Spouting bed blenders and fluidized bed blenders have also been employed. In the prior art, UO.sub.2 powders have primarily been blended with mechanical tumble-type blenders such as disclosed in U.S. Pat. No. 3,825,230 to Frye et al. This blender has frequently failed to produce blended batches meeting UO.sub.2 powder homogeneity specifications. Failure to meet homogeneity specifications is thought to occur because of stagnant or deadzones within the blender and segregation problems during discharging.
Furthermore, at least a thirty minute blending cycle is required with this type of blender. The length and the nature of the mechanical blending process causes the grinding of the powder into smaller particle sizes which is a great disadvantage in later manufacturing steps when the powder is pressed into UO.sub.2 fuel pellets. The physical layout of the mechanical tumble blender also limits the charging and discharging flow rates. In addition to these blending problems large mechanical tumble blenders present a physical safety hazard due to the large rotating mixing chamber, which in practice is generally about six feet in diameter.
Of the two major types of blenders presently in use in which a mixing gas is employed, spouting bed blenders such as the one shown in U.S. Pat. No. 2,786,280 to Gishler et al have not been adopted for the blending of UO.sub.2 powders. This is due to the violent action of gas jets penetrating and spouting from the top of the bed causing an excessive loss of UO.sub.2 powder through entrainment with the fluidizing gas.
The other major type of prior art blender, the bubbling-bed fluidized bed blenders, having a simple planar array of either upwardly or downwardly directed fluidizing orifices, have also been unable to meet product homogeneity specifications. This is due to stagnant or deadzones that exist at the bottom of the fluidized bed between the gas orifices. A discussion of the design considerations involved in designing a prior art bubbling-bed fluidized bed blender of this type, including a consideration of particle properties, particle size distribution, vessel geometry, superficial gas velocity and circulation patterns, is found in "Fluidization and Particle Fluid Systems" by Frederick A. Zenz and Donald F. Othmer, Reinhold Chemical Engineering Series, Reinhold Publishing Corporation, New York, 1960. Design considerations for possible grid designs may be found in "Fluidization" by J. F. Davidson and D. Harrison, Academic Press, London, 1971.
Referring to FIGS. 1 and 2 the operation of a prior art bubbling-bed fluidized bed blender having either downwardly and upwardly directed fluidizing orifices, respectively, is illustrated. Both the blenders of FIG. 1 and FIG. 2 have a flat or gently sloped bottom wall 1 with a drain 2 disposed at the base of side wall 3. In the case of FIG. 1, a planar array of downwardly directed fluidizing gas orifices 4 is provided and in the case of FIG. 2 a planar array of upwardly directed fluidizing gas orifices 5 is provided. In both cases the orifices supply fluidizing gas at a velocity sufficient to cause bubbles of fluidizing gas to rise through the particulate matter contained in the blender in a manner well-known in the prior art. Circulatory patterns created in the particles of the bed by those rising bubbles are illustrated by the arrows 6. The problem with prior art bubbling-bed fluidized bed blenders presented in these Figures is that shaded stagnant or deadzones 7 are created on the bottom wall 1 between the orifices 4 in FIG. 1 and between the orifices 5 in FIG. 2. These deadzones make it difficult for prior art bubbling-bed fluidized bed blenders to meet product homogeneity specifications for the blending of UO.sub.2 powder and make it difficult to completely drain the bed of the blender after the blending process has been completed.